How to add a crust of crystals to this object?
You can use a Particle system.


Model the crystals, i would recomend 5-6 different variations
Make them a group/collection(2.8)
Create a particle system.
Change the Render from eg. Halo to "Collection"
select your collection with the models
add a VertexWeigth in the Vertex settings as ammount
go to weigth paint and paint on where you need them to be.


You can also create a weigth paint for size to controll it